1. Field of the Invention
The present invention relates to a phase-shifted distributed feedback laser device in which an effective refractive index of a laser active layer is discontinuous, and an improvement in a method of manufacturing the same.
2. Description of the Related Art
A DFB-LD (Distributed FeedBack-Laser Diode) plays an important role in long-distance large-capacity optical communication. A characteristic feature of this DFB-LD lies in a structure (i.e., a diffraction grating) in which a refractive index periodically changes in an optical waveguide. However, when cleavage of crystals is used in a reflecting mirror, it is difficult to oscillate the DFB-LD in a single longitudinal mode because the phase of a diffraction grating at the end of the DFB-LD has an adverse effect on oscillation in the single longitudinal mode.
Therefore, the following proposals have been made for the DFB-LD in terms of a device structure.
First, a .lambda./4-shifted DFB laser in which the phase of a diffraction grating is shifted by .pi. (.pi./2 as the phase of an oscillation frequency) in a central portion of a laser resonator has been developed (this laser is described in, e.g., Electronics Letters vol. 20, NO 24, 1984, pp 1,008 to 1,010). According to this proposal, laser light reliably oscillates in a single longitudinal mode. Second, an effective refractive index type .lambda./4-shifted DFB laser in which the width of a waveguide is changed in a central portion of a laser resonator to change the effective refractive index of an active medium has been developed. A characteristic feature of this proposal is that because the phase of waveguide light changes in a portion where the width of the waveguide changes, an effect similar to that obtained when a diffraction grating is shifted can be equivalently obtained (this laser is described in, e.g., IEEE JOURNAL OF QUANTUM ELECTRONICS. VOL. QE-23, NO 6JUNE 1987 pp 804 to 814).
In the .lambda./4-shifted DFB laser in which the phase of a diffraction grating is shifted, however, when a coupling coefficient .kappa. indicating a feedback quantity of a light wave is large, light reciprocated in a resonator is easily concentrated on a phase shift portion A of the diffraction grating, as shown in FIGS. 1A and 1B. Therefore, the performance of the laser is largely degraded, e.g., hole burning in the axial direction results, causing saturation of an optical output (this is described in, e.g., IEEE JOURNAL OF QUANTUN ELECTRONICS. VOL. 24, NO 11, November 1988). On the contrary, when the coupling coefficient .kappa. is small, light reciprocated in the resonator is easily concentrated on end faces B of the diffraction grating, as shown in FIGS. 2A and 2B. Therefore, it is very difficult to adjust the coupling coefficient .kappa., e.g., a threshold current rise or no satisfactory suppression ratio with an adjacent mode. Referring to FIGS. 1A and 2A, reference numeral 1 denotes an InP substrate; 2, a waveguide layer; 3, an active layer; 4, a cladding layer; 5, a contact layer; 6, electrodes; and 7, AR films.
In addition, in the DFB-LD in which the width of a waveguide is changed, concentration of light is decreased in a region where the phase is shifted. However, since even a waveguide having a uniform width is difficult to form, it is very difficult to control the width, the shape, or the like of a region where the phase is shifted. This proposal, therefore, is not useful unless a control apparatus capable of precisely controlling the width of a waveguide is developed. Furthermore, the far field pattern of output light undesirably large projections.
The following proposals have been made for the DFB-LD in terms of a manufacturing method.
First, a method of using both negative and positive resists in order to shift the phase of a diffraction grating has been developed (this method is described in, e.g., Electronics Letters vol. 20, NO 24, 1984, pp 1,008 to 1,010). Second, a method of using a phase shift film has been developed (this method is described in, e.g., Electronics, Information, and Communication Society Research Report OQE86-150). However, neither of the above methods can provide a DFB-LD with a high yield. In addition, these methods of shifting the phase of a diffraction grating have drawbacks in that, e.g., a step is formed in a phase shift portion of the diffraction grating or the shape of the diffraction grating is changed in the shift portion.
In addition to the above proposals, the following proposals have been made for the DFB-LD.
First, a device in which the structure of a waveguide is changed to equivalently shift a phase has been developed. As a practical example of this proposal, a DFB-LD in which the thickness of the waveguide layer 2 of the distributed feedback laser device shown in FIG. 1A or 2A is changed is known (this device is described in, e.g., Published Unexamined Japanese Patent Application No. 61-88584). However, the DFB-LD in which the thickness of the waveguide layer is changed has the following drawbacks. When a waveguide layer is grown after grooves are formed in a substrate, a diffraction grating formed on the substrate is flattened by a conventional etching method. For this reason, no diffraction grating is present in a region where the phase is shifted. In addition, crystal orientation in a region where the diffraction grating is present is different from that in other flat regions. For this reason, various problems are posed, e.g., a change in crystallinity in the waveguide layer, nonuniform crystal growth, or formation of a step in the active layer. Furthermore, since control of the thickness of the grown layer is difficult, it is impossible to shift the phase by a desired making this structure impractical.
A method of forming a diffraction grating after a waveguide layer is grown, contrary to the above method, is known. In this method, an amount of phase shifting can be easily controlled. However, since the thickness of the waveguide layer changes, the thickness of a photoresist used to form the diffraction grating partially changes. As a result, the shape or depth of the diffraction grating cannot be maintained constant, or no diffraction grating is formed at all.
As a method of obtaining a small spectral line width, a method of forming a plurality of regions where a phase is shifted in a resonator has been proposed (this method is described by, e.g., T. Kimura et. el., Electron. lett. vol 23, pp 1,014, 1987). This method is suited to communication of coherent light. In order to realize this method, the above method of shifting a diffraction grating can be applied. However, since hole burning is caused in a region where the diffraction grating is shifted, a spectral line width is increased to be larger than a theoretical value.
On the other hand, the coupling coefficient .kappa. in a portion where the thickness changes is affected by an electric field distribution in a waveguide mode and therefore is different from the coupling coefficient .kappa. in other regions. Controlling the shape or depth of the diffraction grating in a region where the thickness changes is useful in that a change in coupling coefficient .kappa. caused by an electric field distribution difference is compensated. In addition, concentration of light in a region where the phase is shifted or a drop in light distribution must be prevented to flatten the light distribution, thereby adjusting the coupling coefficient .kappa.. However, neither a device nor a manufacturing method satisfying these requirements are presently available.